1. Field of the Invention
The invention relates to a method for simultaneously slicing at least two cylindrical workpieces into a multiplicity of wafers by means of a multi wire saw.
2. Background Art
Multi wire saws are used for example for slicing cylindrical mono- or polycrystalline workpieces of semiconductor material, for example silicon, simultaneously into a multiplicity of wafers in one working step. The production of semiconductor wafers from cylindrical semiconductor material, for example single crystal rods, places exacting requirements on the sawing method. The sawing method ideally ensures that each sawed semiconductor wafer should have two surfaces which are as plane as possible and lie parallel to one another. The throughput of the multi wire saw is also of great importance for economic viability.
In order to increase the throughput, it has been proposed for a plurality of workpieces to be simultaneously clamped into the multi wire saw and sliced in one working step. U.S. Pat. No. 6,119,673 describes the simultaneous slicing of a plurality of cylindrical workpieces, which are arranged coaxially behind one another. To this end a conventional multi wire saw is used, a plurality of workpieces each adhesively bonded on a sawing bar being fixed with a certain spacing in a coaxial arrangement on a common mounting plate, clamped with it into the multi wire saw and sliced simultaneously. This creates a number of stacks of wafers, which are still fixed on the mounting plate, corresponding to the number of workpieces. After the slicing, separating plates are placed loosely into the spaces between the stacks of wafers, in order to prevent the wafers of the various stacks from being confused. This is of great importance since the wafers produced from different workpieces will generally be further processed in different ways and/or the workpieces may have different properties, specified by the customer to which the wafers will be delivered. It is therefore necessary to ensure that all wafers produced from a workpiece intended for a certain customer or a certain order are further processed together, but processed separately from wafers produced from other workpieces.
After the various wafer stacks have been demarcated by separating plates, the mounting plate is immersed in a basin of hot water so that the wafers connected to the mounting plate via the sawing bar hang below the mounting plate. The hot water dissolves the cement bond between the wafers and the sawing bars, so that the detached wafers fall into a wafer carrier placed at the bottom of the basin. The various wafer stacks, which are subsequently contained in the wafer carrier, are separated from one another by the previously introduced separating plates.
The method disclosed in U.S. Pat. No. 6,119,673 for demarcating the various stacks of wafers has the disadvantage that the wafer stacks are not secured against lateral tilting (as can be seen in FIG. 8(C) of U.S. Pat. No. 6,119,673) and the edges, which are very sharp after the slicing, consequently fracture. Placement of the separating disks according to the method described in this application is furthermore very difficult, since the separating disks must be inserted between the labile separated wafer stacks and held in their position while the wafer stack is lowered into the wafer carrier from above. If a separating plate comes in contact with a wafer stack during this process, then wafers may break off from the sawing bar, fall into the wafer carrier from a relatively large height and therefore be damaged or destroyed.
U.S. Pat. No. 6,802,928 B2 describes a method in which dummy pieces with the same cross section are adhesively bonded onto the end surfaces of the workpiece to be sliced, sliced with the workpiece and then discarded. This is intended to prevent the resulting wafers from fanning out at the two ends of the workpiece during the end phase of the slicing, and therefore to improve the wafer geometry. This method has the crucial disadvantage that some of the gang length, which is limited by the dimensions of the multi wire saw, is used for slicing the “unused” dummy pieces and is therefore not available for the actual production of the desired wafers. Furthermore, the provision, handling and adhesive bonding of dummy pieces is very elaborate. Both lead to a significant reduction in economic viability.
Also in the method described in U.S. Pat. No. 6,119,673 for simultaneously slicing a plurality of workpieces in a multi wire saw, the gang length of the multi wire saw often cannot be utilized optimally since the workpieces to be sliced have very different lengths owing to the way in which they are produced. This problem arises particularly when the workpieces consist of monocrystalline semiconductor material, since the known crystal pulling processes only permit certain usable lengths of the crystals or it is necessary to cut the crystals and produce test specimens at various positions of the crystal in order to control the crystal pulling process. Furthermore, various types of semiconductor wafers with different properties (which for the most part are already defined by the crystal from which the wafers are produced) are usually fabricated in the same plant for a plurality of customers, in which case different delivery deadlines need to be complied with.